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Evidence from Past Climates

Introduction

Although mankind may be changing the climate through the emission of greenhouse gases, climate also changes in response to natural factors, including variations in sunlight, changes in ocean circulation and volcanic eruptions. Throughout the Earth's history climate has fluctuated between periods of relative warmth and relative cold. Palaeoclimatology is the study of climate and climate change prior to the period of direct (instrumental) measurements. Instrumental records of temperature and other climatic variables span only a tiny fraction of the Earth's climatic history, and so provide an inadequate perspective on climatic variation and the evolution of the climate today and in the future. A longer perspective on climate variability can be obtained by the study of natural phenomena which are climate-dependent. Such phenomena provide a record of the climate. Palaeoclimatology can provide clues as to how the global climate may change in the future.

Many natural systems are dependent on climate; from these it may be possible to derive palaeoclimatic information. Such records of climate contain a climatic signal. Deciphering that signal is often a complex business. Four types of non-instrumental record are commonly analysed for past climate variations: historical records; glaciological records (e.g. ice cores); biological records (e.g. dendroclimatology or tree ring analysis); and geological records (e.g. ocean sediments).

Historical Records

Historical records have been used to reconstruct climates dating back several thousands of years. Historical data can be grouped into three major categories. First, there are observations of weather phenomena per se, for example the frequency and timing of frosts, or the occurrence of snowfall. Secondly, there are records of weather-dependent environmental phenomena, termed parameteorological phenomena, such as droughts and floods. Finally, there are phenological records of weather-dependent biological phenomena, such as the flowering of trees, or the migration of birds. Major sources of historical palaeoclimate information include: ancient inscriptions; annals and chronicles; government records; estate records; maritime and commercial records; diaries and correspondence; scientific or quasi-scientific writings; and fragmented early instrumental records.

Ice Cores

As snow and ice accumulates on ice caps and sheets, it lays down a record of the environmental conditions at the time of its formation. Information concerning these conditions can be extracted from ice and snow that has survived the summer melt by physical and chemical means. Palaeoclimate information during the Ice Age (last 130,000 years) has been obtained from ice cores by three main approaches. These involve the analysis of: a) the (isotopic) composition of the water in the ice; b) dissolved and particulate matter in the ice; and c) the physical characteristics of the firn and ice, and of air bubbles trapped in the ice.

Dendroclimatology

The study of the relationships between annual tree growth and climate is called dendroclimatology. Dendroclimatology offers a high resolution (annual) form of palaeoclimate reconstruction for the last few thousand years. Whenever tree growth is limited directly or indirectly by some climate variable, usually temperature or rainfall, and that limitation can be quantified and dated, dendroclimatology can be used to reconstruct some information about past environmental conditions. There are several subfields of dendroclimatology associated with the processing and interpretation of different tree-growth variables. Such variables include tree-ring width, tree ring density, and chemical or isotopic variables.

Ocean Sediments

Billions of tonnes of sediment accumulate in the ocean basins every year, and this may be indicative of climate conditions near the ocean surface or on the adjacent continents. Sediments are composed of both biogenic (organic) and terrigenous (inorganic) materials. The biogenic component includes the remnants of planktonic (surface ocean-dwelling) and benthic (deep-water or sea floor-dwelling) organisms which provide a record of past climate and oceanic circulation. Such records may reveal information about past surface water temperatures, salinity, dissolved oxygen and nutrient availability. By contrast, the nature and abundance of terrigenous materials provides information about continental humidity-aridity variations, and the intensities and directions of winds. Ocean sediment records have been used to reconstruct palaeoclimate changes over a range of time scales, from thousands of years to millions and even tens of millions of years in the past.

Glacials and Interglacials

Numerous palaeoclimatic records have been used to reconstruct climate variations during the last several hundred thousand years. Such reconstructions have demonstrated that the global climate has fluctuated between states of relative warmth (interglacial) and relative frigidity (glacial or ice age). It has been suggested that the cause of these climate variations lies in the changes in the Earth's orbit around the Sun over similar time periods, amplified by secondary climatic feedbacks including ocean circulation changes, ice sheet growth/decay and atmospheric carbon dioxide variations. Currently, the Earth's climate is experiencing an interglacial episode.

Antarctic Climate in the last 150,000 Years


The Little Ice Age

Tree rings and ice cores have been extensively used to reconstruct climate during the last 1,000 years. Beginning about 1450 A.D. an interval of relative frigidity (within the current longer term interglacial) often called the Little Ice Age has been identified. The term has been used to describe an epoch of renewed glacial advance. Although many regions of the world experienced cooling during the period 1450 to 1890 A.D., its use has been criticised because it could not conclusively be considered an event of global significance

There is considerable evidence that the Little Ice Age consisted of two main cold stages of about a century's length. These occurred in the seventeenth an nineteenth centuries, with relative warmth arising in the sixteenth and eighteenth centuries. Glaciers advanced in Europe, Asia and North America, whilst sea ice in the North Atlantic expanded with detrimental effects for the colonies of Greenland and Iceland.

Conclusion

This fact sheet has reviewed some of the methods and techniques used to reconstruct palaeoclimates from non-instrumental data sources. Whenever an indicator is chosen to represent some aspect of the climate, one must be sure that there exists a physical basis for the choice of that indicator, i.e. variations in the record of that indicator truly reflect variations in that aspect of the climate one is attempting to measure. The use of palaeoclimate information has revealed evidence for past fluctuations in climate over different time scales.